Severe haemolytic anaemia developed in a 33 year old patient after the mitral valve was replaced with a St Jude medical prosthesis. This was the patient's third thoracotomy. She had already had a mitral commissurotomy and a mitral valve bioprosthesis. The patient had an E+ antibody to red blood cells as well as a paraprosthetic leak. The haemolysis became less severe once the population of E+ red cells was completely haemolysed. However, the patient continued to require transfusions to remain out of heart failure. Once the valve was replaced, the haemolysis subsided. Fulminant haemolysis after mitral valve replacement is rare. When it does occur, a paraprosthetic leak should be suspected. Other causes of haemolysis, however, must also be considered and these may contribute to the severity of haemolysis.